Beaten so badly he repeatedly tried to kill himself, when Chen Keyun signed his confession to bombing a Chinese Communist party office he was not even sure what it said.

But with it he was convicted and sent to prison -- and only cleared 12 years later after all-consuming lobbying by his family.

His case, and unknown others that remain uncorrected, reflect widespread abuses in China's legal process where police routinely coerce admissions of guilt and courts have a near-perfect conviction rate.

"The whole thing was a joke," said Chen, 61, in his first interview with foreign media, lifting his forearms to show marks left from detention where he said he was hung by shackles, chained into stress positions, force-fed water and made to vomit.

"They used their power to extract a confession from an innocent person -- for their own selfish interests," he told AFP, sitting in his lawyer's office in Fuzhou, the capital of the eastern province of Fujian.

"I am very angry."

China's politically controlled courts -- the constitution enshrines the leading role of the Communist Party -- find 99.9 percent of defendants guilty, according to the latest US State Department human rights report on the country.

No official Chinese figures are available and the incidence of wrongful decisions is not known, but a series of long-sought reversals have been reported in domestic media this year.

Rights groups say forced confessions are common, and Supreme Court vice president Shen Deyong warned in May that wrongful convictions posed an "unprecedented challenge" for China's judiciary.

"It might even be said that the likelihood of wrongful convictions occurring is relatively high," he wrote in an essay.

Fujian courts declared Chen, his driver, his wife and two others not guilty earlier this year and offered them 4.3 million yuan ($700,000) in compensation in September, though they are fighting for more.

The five were convicted for a 2001 bombing that killed a man at a corruption investigation bureau in Fuqing.

A disgruntled subordinate had reported Chen, formerly a government official, as having a grievance with the bureau.

Under duress to reveal their supposed explosive suppliers, Chen's driver Wu Changlong, 39, named his sister's ex-husband Du Jiesheng, 48, who in turn implicated work acquaintance Tan Minhua, today still only about 30.

When Du resisted signing his confession, he said, officers drove him to a mountain "and were going to push me off" until he gave in.

He too tried to kill himself several times in custody -- until guards began to follow him even to the toilet and made him wear a helmet to stop him cracking his skull open.

Chen and Wu were handed suspended death sentences in 2001 and the others received prison terms as long as 10 years, reduced on appeal to between three and seven.

"Confessions are cheap"

The use of forced confessions, along with pressure to solve cases, are the root causes of wrongful convictions, said Margaret Lewis, a Chinese law expert at Seton Hall University in New York.

"Confessions are cheap. There's a strain on resources, a lot of pressure for job-evaluation reasons to get cases cleared, and they push it though," she said.

"It goes on to the courts and traditionally there's been very little if any incentive for anyone further down that process... to challenge the police."

China's Communist chief Xi Jinping, who has repeatedly pledged to improve people's livelihoods, last December urged officials to "let the people experience fairness and justice in every legal case".

New Supreme Court president Zhou Qiang -- who unlike his predecessor has legal training -- made similar calls in July.

But New York University law professor Ira Belkin is sceptical of the political rhetoric and instead credits a minority of judges and lawyers for steadily pushing for reform.

"It's one thing to talk about it. It's another thing to really allow lawyers to reopen a lot of cases," he said. "These cases are embarrassing."

Many only succeed with bottom-up efforts backed by media attention or irrefutable evidence -- as when a "murder victim" in the central province of Henan turned up alive in 2010, a decade after a fellow villager had been jailed for killing him.

"The courts did not take the initiative to correct these cases but were forced to correct them," Wang Yongjie, a visiting professor at China University of Political Science and Law in Beijing, said in an email.

Relatives of the Fuqing group lobbied relentlessly at great expense, even waylaying senior Chinese officials when they went overseas.

Wu's sister petitioned authorities in Beijing so often that she herself was locked away for a year, and the family borrowed more than 700,000 yuan to cover legal and other costs.

Their efforts finally drew enough attention from the media and national leadership to pressure provincial leaders to act, some in the group said. The Fujian high court reheard the case and declared them not guilty.

Yet the five are still fighting -- for a high-profile apology, eight times more compensation than they were granted, and accountability for the real culprits and the officials who beat and convicted them.

Rebuilding life is another struggle in itself. Wu, a wiry man with a stiff gait, said injuries from police have left him deaf in one ear and hard-pressed to sleep through the night, or find work.

"If they don't resolve this properly, then I will appeal all the way up to the Supreme Court. I will sue the judge that sentenced me and the head judge of that court," he said.

"Everything that we went through, no amount of money can compensate."